It has been known for some time that undesirable formation of polymeric by-products in the preparation of Diels-Alder adducts could be prevented by adding certain polymerization inhibitors. Such formation of by-products is thus more or less extensively impeded, depending on whether the process is conducted discontinuously or continuously.
British Pat. No. 1,492,901 discloses the most recent and heretofore most advantageous prior art process and also provides a discussion of the earlier state of the art. The patent teaches the use of certain p-phenylenediamine compounds as polymerization inhibitors. They are to be added preferably in an amount of 50-500 ppm (weight/weight), based on the sum total of diene component and dienophilic component. The cost of manufacturing these p-phenylenediamine compounds is comparatively high, which makes their use a burden on the economics of the disclosed process.
A need therefore continues to exist for more economical inhibitors of polymeric by-products which nevertheless are compatible with the reactants, products and conditions of the Diels-Alder reaction.